Radio receiving system



Nov. 30 1926.

S. CABOT RADIO RECEIVING SYSTEM Filed NOV. 19, 1924 Patented No...30,1926.-

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BEWALL QAIBOT, OI BBOOKLINE, MASBACHUSE'I'EB.

RADIO RECEIVING SYSTEM.

Application fled November 19, 1824. Serial No. 750,784.

The present invention which is an improvement on the system described 1nmy application Serial No. 734,205, filed August 26, 1924, relates toradio-receiving systems 5 of the radio-frequency amplifier type inwhicha three-electrode vacuum tube is interposed between an oscillatoryrece1v1ng circuit and a second oscillatory circuit, the

said circuits not being exteriorly coupled or interlinked, but coupledor interlinked only by the elements of said tube. In systems of thistype if capacity only is varied for tuning both circuits, any localoscillations that may be created in the system by controlllng the secondoscillatory circuit will be of greater intensity on short wave lengths,and if inductance only is varied, such local oscillations will be ofgreaterintensity on long wave lengths. However, if the inductance of onecircuit is kept constant and the capacity thereof only is varied. andthe capacity of the other circuit is kept constant, or approximatelyconstant, and the inductance thereof only is varied, suchlocaloscillations. so created by such control, will be of substantially equalintensity for all wave lengths within the range of the system.

I have discovered that certain advantages result from connecting theends of the input circuit of the amplifier tube to the receivinginductance, for example a receiving loop, at one of the terminalsthereof and at a point intermediate said terminals, respectively,instead of connecting said input circuit to the terminals of saidinductance, the exact location of said point intermediate the terminalsof said inductance depending upon the electromagnetic and resistanceconstants of the several elements of the system and being bestdetermined experimentally, al-

though under certain conditions I have found that the best results areobtained when said point is approximately midway between the terminalsof said inductance.

Some of the advantages obtained by connecting the filament of athree-electrode tube to one terminal of the receiving inductance or loopand the grid of said tube thereto at a point intermediate the terminalsthereof, for instance, at a point midway between said terminals are (1)an increase in the selectivity of the system, (2) an increase in thewave-length range of the system for .a given condenser, 3) the obtaininof greater stabilit of ad ustment to the oating point or t e point atwhich the system 'lS just on the verge of creating local oscillations,this being the condition in which the system is most sensitive 'tofeeble electrical oscillations, and (4) the ability to reduce theintensity of any local oscillations that may be'created in the system tozero or to as low a value as desired and at the same time provide alarge amount of regeneration under control at all wave lengths withinthe tuning range. c

The object of my invention is to provide a radio-receiving systemwhereby the advantages above enumerated may be realized in practice.

With the foregoing object in view my invention in its broadest aspectessentially consists in a receiving inductance and a vacuum tube havingthe ends of its input circuit connected thereto at one of the terminalsthereof and at a point intermediate said terminals, respectively, suchintermediate point being, under certain circumstances, approximatelymidway between said terminals.

- My invention further consists in a fixed receiving inductance, suchfor example, as a loop, with a variable condenser connected across theterminals thereof, a vacuum tube having the ends of its input circuitconnected, respectively, to one terminal of said inductance and a pointintermediate said terminals in combination with a fixed condenser and avariable inductance associated with the plate circuit of said tube, andmechanical means for effecting the simultaneous variation of thecapacity of said variable condenser and the value of said inductance,whereby any local oscillations that may be created in the s stem bychanging the capacity of a third independent condenser (which may be inmultiple with either of the condensers aforesaid), will be ofapproximately equal intensity for all wave lengths within the range ofthe system.

My invention consists also in a method of tuning radio-receiving systemsof the radiofrequency amplifier type to the floating point, orthe pointof maximum sensitiveness for any wave length within therange of saidsystem which consists, first, in simultaneously changing in oppositedirections the impedance of the input and output circuits of a vacuumtube as the wave length varies, for example, reducing the impedance ofthe input circuit by increasing the capacity thereof and increasing theimpedance of the output circuit by increasing the inductance of saidoutput circuit, whereby any local oscillations that may be created insaid system by varying a separate adjustable controlling means, such asa condenser associated with either circuit. will be of substantiallyuniform intensity for any wavelength within the range of saidsystem:and, second, reducing the intensity of such local os-'- cillations tozero" or to as low a value as desired by an experimentally-determinedfixed adjustment of the impedance of the input circuit.

In the drawing accompanying and forming a part of this specification thefigure is a diagram of a radio-receiving system embodying my invention.

In the particular diagram selected for" more fully disclosing myinvention, 1 is a receiving inductance, shown mJthe present instance asa loop having fixed inductance,

across the terminals 2, 3 of which is con nected 'a variable condenser4. The filament 5 of a three-electrode amplifying vacuum tube isconnected to said loop at the terminal 3,thereo f. The grid 6 of saidtube is connectedto said loop at the'point 7 which is intermediate theterminals 2, 3 of said loop, said point 7 being best. determinedexperimentallyand its location depending upon the electromagnetic andresistance constants of the several elements of the svstem.

I have found that under certain conditions the point 7 may withadvantage be approximately midway between the terminals 2. 3.

A fixed condenser 8 and a variable induc-- tance 9 which may be atransformer vari-v condenser 4 and inductance 9 are shown as mounted ona shaft 11 rotatable by the knob 12. A second variable condenser 13.shown in the present instance as connected in multiple with the fixedcondenser 8. although it may be connected across the terminals of.

the variable condenser 4, is employed to compensate for inequalities andensure that for every position of the knob 12 the product of inductanceby capacity in the input circuit shall be equal to the product ofinductance by capacity of the output circuit, to govern regeneration andthe creation of local oscillations in the system, and to adjust thesystem to the floating point aforesaid.

By maintaining the inductance of the input circuit constant and varyingonly the capacity 4 thereof, and maintainin the capacity 8 associatedwith the circult of the plate 10 constant or approximately constant, andvarying only the inductance 9 of such plate circuit, any localoscillations that may be created in the system by varying the capacityof the condenser 13 will be of substantially equal intensity for allwave lengths within the range of the system, such intensity beingcontrollable and reducible to zero or to as low a value as desired byexperimentally selecting the location of the intermediate point 7 on theloop 1.

, A suitable detector such as the tube 14 having a telephone receiver15, or the like, in its plate circuit, is associated with the variableinductance 9 and condenser 10, although it will be understood ofcoursethat said tube-14 may be an amplifying tube or one of a series ofamplifying tubes. The battery 16 is employed to heat the filaments ofthe-two tubes and the battery 17 for supplying energy to the outputcircuits thereof. Assuming that the point 7 is midway between theterminals 2, '3 of the receiving loop, the inductance of the gridcircuit of the first tube is only approximately onefourth of theinductance of the loop because in such case the grid or input circuit ofsaid tube 6,7, 3, 5. contains only one-half as many turns as the loop.Therefore the. reactance of the variable condenser 4 on the grid circuitis only one-fourth as great as it would be if the grid were connected tothe terminal of the loop 2, instead of the point 7 midway between theterminals 2, 3. If follows that a given change in the capacity of thecondenser 4 will effect a larger proportional change in the apparentreactancebetween the grid 6 and filament 5 than if said grid wereconnected to said terminal 2. It is for this reason that the connectionof said grid to the loop at the point 7 instead of to the terminal 2 ofthe loop greatly increases the selectivity of the system.

When the output circuit of the first tube is attuned to the frequency ofthe oscillations to be received and "amplified, the gridplate capacityis about ten times larger than its normal value. being equal to suchnormal value multiplied by (n+1) where p is the amplification factor.When the grid 6 is connected to the loop at the point 7, assumed to bemidway between the loop terminals 2 and 3, the effect of the grid-platecapacity on the loop condenser 4 is only about onefourth of that whichwould be exerted thereon if said grid were connected to the loop at itsterminal 2. It follows therefore that when the grid is connected to theloop at the point 7 the eflective capacity of the loop is less for thezero setting of the variable condenser 4 than when the grid is connectedto the terminal 2. For a condenser 4 of given capacity, the system canbe made resonant to oscillations of higher frequency and shorter wavelength by the mode of connection above described and therefore the wavelength range of the system is increased.

The tube 5, 6, 10 will oscillate when its plate-circuit is attuned to ahigher frequency than its grid circuit, and such oscillations may bestopped by increasing the capacity or a 2 thereof.

the inductance of said plate circuit. The tube may be made to oscillateby reducing the frequency to which the grid circuit is attuned, and suchoscillations may be stopped by increasing the frequency of said gridcircuit. The apparent grid-to-plate capacity of the tube varies with theplate voltage, and as such voltage increases at resonance, it followsthat the grid-plate capacity increases when the plate circuit is attunedto the frequency of the oscillations which are to be received andamplified. Such increase of grid-plate capacity is effective to increasethe apparent capacity of the loop and thereby reduce the frequency towhich it is attuned, thereby causing the tube to oscillate and creatinga condition which is inherently unstable. However, by virtue of theconnection of the grid to the loop at the point 7 instead of to theterminal 2, the effect of the grid-plate capacity on the loop condenser4 is only one-fourth of the effect that would be produced if the gridwere connected to the loop at the terminal For this reason it ispossible to obtain greater stability of adjustment to the floating pointor the point at which the tube is just on the verge of oscillating.

The advantages above recited are quite independent of the adjustment ofthe electromagnetic constants whereby the regeneration may be governedto be of uniform intensity for all wave lengths within the range of thesystem, viz, the variation of the capacity 4. the inductance 1 remainingconstant, and the variation of the inductance 9, the capacity of thecondenser 8 remaining constant or substantially constant.

However the fourth advantage, viz, the ability to control, that is,minimize or suppress, the creation of oscillations in the loop circuit,is dependent upon obtaining regeneration of substantially equalintensity for all wave lengths within the range of the system.

It will be obvious that when the system is designed as above set forthfor such substantially equal intensity of regeneration, the intensity ofoscillations created in the system may be reduced to zero or to as low avalue as desired, by reducing the impedance of the input circuit of thetube as by moving the point 7 closer to the terminal 3; and that suchintensity may be increased by increasing the impedance of said inputcircuit as by moving the point 7 farther away from the terminal 3 andcloser to the terminal 2. When the received energy is small it issometimes desirable to create feeble oscillations in the loop circuit 1,4, by moving the point 7 closer to the terminal 2, and in the case ofstronger waves, the intensity of such oscillations may with advantage bereduced to zero and the tendency to oscillate suppressed by moving saidpoint 7 closer to the terminal 3.

Having thus described an illustrative embodiment of my invention withouthowever limiting the same thereto, what I claim and desire to secure byLetters Patent is The method of tuning a radio-receiving system of theradio-frequency amplifier type to the point of greatest sensitivenessfor any wave length within the range of said system which consists,first, in simultaneously changing in opposite directions, the impedanceof the input and output circuits of a vacuum tube as wave length varieswhereby any local oscillations that may be created in said system willbe of substantially uniform intensity for any wave length within therange of said system. and, second, reducing the intensity of such localoscillations to as low a value as desired.

In testimony whereof, I have hereunto subscribed my name this- 17th dayof November, 1924.

SEWALL GABOT.

